.TH deadlock 8  "2017-02-01" "USER COMMANDS"
.SH NAME
deadlock \- Find potential deadlocks (lock order inversions)
in a running program.
.SH SYNOPSIS
.B deadlock [\-h] [\--binary BINARY] [\--dump-graph DUMP_GRAPH]
.B          [\--verbose] [\--lock-symbols LOCK_SYMBOLS]
.B          [\--unlock-symbols UNLOCK_SYMBOLS]
.B          pid
.SH DESCRIPTION
deadlock finds potential deadlocks in a running process. The program
attaches uprobes on `pthread_mutex_lock` and `pthread_mutex_unlock` by default
to build a mutex wait directed graph, and then looks for a cycle in this graph.
This graph has the following properties:

- Nodes in the graph represent mutexes.

- Edge (A, B) exists if there exists some thread T where lock(A) was called
and lock(B) was called before unlock(A) was called.

If there is a cycle in this graph, this indicates that there is a lock order
inversion (potential deadlock). If the program finds a lock order inversion, the
program will dump the cycle of mutexes, dump the stack traces where each mutex
was acquired, and then exit.

This program can only find potential deadlocks that occur while the program is
tracing the process. It cannot find deadlocks that may have occurred before the
program was attached to the process.

This tool does not work for shared mutexes or recursive mutexes.

Since this uses BPF, only the root user can use this tool.
.SH REQUIREMENTS
CONFIG_BPF and bcc
.SH OPTIONS
.TP
\-h, --help
show this help message and exit
.TP
\--binary BINARY
If set, trace the mutexes from the binary at this path. For
statically-linked binaries, this argument is not required.
For dynamically-linked binaries, this argument is required and should be the
path of the pthread library the binary is using.
Example: /lib/x86_64-linux-gnu/libpthread.so.0
.TP
\--dump-graph DUMP_GRAPH
If set, this will dump the mutex graph to the specified file.
.TP
\--verbose
Print statistics about the mutex wait graph.
.TP
\--lock-symbols LOCK_SYMBOLS
Comma-separated list of lock symbols to trace. Default is pthread_mutex_lock.
These symbols cannot be inlined in the binary.
.TP
\--unlock-symbols UNLOCK_SYMBOLS
Comma-separated list of unlock symbols to trace. Default is
pthread_mutex_unlock. These symbols cannot be inlined in the binary.
.TP
pid
Pid to trace
.SH EXAMPLES
.TP
Find potential deadlocks in PID 181. The --binary argument is not needed for \
statically-linked binaries.
#
.B deadlock 181
.TP
Find potential deadlocks in PID 181. If the process was created from a \
dynamically-linked executable, the --binary argument is required and must be \
the path of the pthread library:
#
.B deadlock 181 --binary /lib/x86_64-linux-gnu/libpthread.so.0
.TP
Find potential deadlocks in PID 181. If the process was created from a \
statically-linked executable, optionally pass the location of the binary. \
On older kernels without https://lkml.org/lkml/2017/1/13/585, binaries that \
contain `:` in the path cannot be attached with uprobes. As a workaround, we \
can create a symlink to the binary, and provide the symlink name instead with \
the `--binary` option:
#
.B deadlock 181 --binary /usr/local/bin/lockinversion
.TP
Find potential deadlocks in PID 181 and dump the mutex wait graph to a file:
#
.B deadlock 181 --dump-graph graph.json
.TP
Find potential deadlocks in PID 181 and print mutex wait graph statistics:
#
.B deadlock 181 --verbose
.TP
Find potential deadlocks in PID 181 with custom mutexes:
#
.B deadlock 181
.B      --lock-symbols custom_mutex1_lock,custom_mutex2_lock
.B      --unlock_symbols custom_mutex1_unlock,custom_mutex2_unlock
.SH OUTPUT
This program does not output any fields. Rather, it will keep running until
it finds a potential deadlock, or the user hits Ctrl-C. If the program finds
a potential deadlock, it will output the stack traces and lock order inversion
in the following format and exit:
.TP
Potential Deadlock Detected!
.TP
Cycle in lock order graph: Mutex M0 => Mutex M1 => Mutex M0
.TP
Mutex M1 acquired here while holding Mutex M0 in Thread T:
.B [stack trace]
.TP
Mutex M0 previously acquired by the same Thread T here:
.B [stack trace]
.TP
Mutex M0 acquired here while holding Mutex M1 in Thread S:
.B [stack trace]
.TP
Mutex M1 previously acquired by the same Thread S here:
.B [stack trace]
.TP
Thread T created by Thread R here:
.B [stack trace]
.TP
Thread S created by Thread Q here:
.B [stack trace]
.SH OVERHEAD
This traces all mutex lock and unlock events and all thread creation events
on the traced process. The overhead of this can be high if the process has many
threads and mutexes. You should only run this on a process where the slowdown
is acceptable.
.SH SOURCE
This is from bcc.
.IP
https://github.com/iovisor/bcc
.PP
Also look in the bcc distribution for a companion _examples.txt file containing
example usage, output, and commentary for this tool.
.SH OS
Linux
.SH STABILITY
Unstable - in development.
.SH AUTHOR
Kenny Yu
